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21 Mar 2009, 23:29

==About a century ago, the Swedish physical scientist Arrhenius proposed a law of classical chemistry that relates chemical reaction rate to temperature. According to the Arrhenius equation, chemical reactions are increasingly unlikely to occur as temperatures approach absolute zero, and at absolute zero (zero degrees Kelvin, or minus 273 degrees Celsius) reactions stop. However, recent experimental evidence reveals that although the Arrhenius equation is generally accurate in describing the kind of chemical reaction that occurs at relatively high temperatures, at temperatures closer to zero a quantum-mechanical effect known as tunneling comes into play; this effect accounts for chemical reactions that are forbidden by the principles of classical chemistry. Specifically, entire molecules can “tunnel” through the barriers of repulsive forces from other molecules and chemically react even though these molecules do not have sufficient energy, according to classical chemistry, to overcome the repulsive barrier.

The rate of any chemical reaction, regardless of the temperature at which it takes place, usually depends on a very important characteristic known as its activation energy. Any molecule can be imagined to reside at the bottom of a so-called potential well of energy. A chemical reaction corresponds to the transition of a molecule from the bottom of one potential well to the bottom of another. In classical chemistry, such a transition can be accomplished only by going over the potential barrier between the wells, the height of which remains constant and is called the activation energy of the reaction. In tunneling, the reacting molecules tunnel from the bottom of one to the bottom of another well without having to rise over the barrier between the two wells. Recently researchers have developed the concept of tunneling temperature: the temperature below which tunneling transitions greatly outnumber Arrhenius transitions, and classical mechanics gives way to its quantum counterpart.

This tunneling phenomenon at very low temperatures suggested my hypothesis about a cold prehistory of life: the formation of rather complex organic molecules in the deep cold of outer space, where temperatures usually reach only a few degrees Kelvin. Cosmic rays (high-energy protons and other particles) might trigger the synthesis of simple molecules, such as interstellar formaldehyde, in dark clouds of interstellar dust. Afterward complex organic molecules would be formed, slowly but surely, by means of tunneling. After I offered my hypothesis, Hoyle and Wickramasinghe argued that molecules of interstellar formaldehyde have indeed evolved into stable polysaccharides such as cellulose and starch. Their conclusions, although strongly disputed, have generated excitement among investigators such as myself who are proposing that the galactic clouds are the places where the prebiological evolution of compounds necessary to life occurred.

21. The author of the passage is primarily concerned with

(A) describing how the principles of classical chemistry were developed(B) initiating a debate about the kinds of chemical reactions required for the development of life(C) explaining how current research in chemistry may be related to broader biological concerns(D) reconciling opposing theories about chemical reactions(E) clarifying inherent ambiguities in the laws of classical chemistry

22. According to the passage, classical chemical reactions and tunneling reactions are alike in which of the following ways?

(A) In both types of reactions, reacting molecules have to rise over the barrier between the two wells.(B) In both types of reactions, a transition is made from the bottom of one potential well to the bottom of another.(C) In neither type of reaction does the height of the barrier between the wells remain constant.(D) In neither type of reaction does the rate of a chemical reaction depend on its activation energy.(E) In both types of reactions, reacting molecules are able to go through the barrier between the two wells.

23. According to the Arrhenius equation as discussed in the passage, which of the following statements about chemical reactions is true?

(A) Chemical reactions are less likely to occur at temperatures close to absolute zero.(B) In some cases the rate of a chemical reaction is related to temperature and in other cases it is not.(C) Chemical reactions frequently occur at a few degrees above absolute zero, but they are very unpredictable.(D) The rate of a chemical reaction depends on many other factors besides temperature.(E) Chemical reaction rate and temperature are not related.

24. The author’s attitude toward the theory of a cold pre-history of life can best be described as

25. The author’s hypothesis concerning the cold prehistory of life would be most weakened if which of the following were true?

(A) Cosmic rays are unlikely to trigger the formation of simple molecules.(B) Tunneling occurs only in a narrow band of temperatures around zero degrees Kelvin.(C) The synthesis of interstellar formaldehyde can be activated by means other than cosmic rays.(D) Simple molecules can be synthesized by means of tunneling.(E) Classical chemical reactions do not occur at temperatures close to absolute zero.

26. Which of the following best describes the hypothesis of Hoyle and Wickramasinghe as it is presented in the passage?

(A) Cosmic rays can directly synthesize complex organic molecules.(B) The galactic clouds are the places where prebiological evolution of compounds necessary to life occurred.(C) Interstellar formaldehyde can be synthesized by tunneling.(D) Molecules of interstellar formaldehyde can evolve into complex organic molecules.(E) Complex organic molecules can be synthesized from stable polysaccharides such as cellulose and starch.

27. Which of the following best describes the organization of the first two paragraphs of the passage?

(A) The author cites a basic principle of classical chemistry and then describes the research from which that principle was developed.(B) The author cites an apparent contradiction to the principles of classical chemistry and then explains the process of a chemical reaction to show there is in fact no contradiction.(C) The author describes the role of heat in chemical reactions and then offers a detailed explanation of its function.(D) The author presents a law of classical chemistry in order to introduce a kind of chemical reaction that differs from it and then explains the essential difference between the two.(E) The author presents the fundamental rules of classical chemistry in order to introduce an explanation of a specific chemical reaction.======
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